The present embodiments relate to a coil system for a magnetic resonance tomography system (e.g., a nuclear spin tomography system).
Magnetic resonance tomography is a method for diagnosing a large number of diseases using imaging. A magnetic resonance tomography system may include at least a background field magnet, a gradient system and high-frequency coils for sending and receiving the high-frequency magnetic resonance signal.
In whole-body magnetic resonance tomography systems, the high-frequency coils may be local field coils that are partially integrated in a patient couch, are to be positioned partially on and around the body of the patient or are disposed in the manner of a helmet around the head. Positioning and removing such local field coils takes time and effort, and the presence of the local field coils in the relatively narrow opening in the magnetic resonance tomography system reinforces the claustrophobic feelings of many patients, contributing to unease.
Such problems may be avoided if the radio frequency coils are positioned in a stationary and invisible manner in the inner wall of the opening in the magnetic resonance tomography system in the manner of a remote body array (RBA). Such a remote body array for whole-body systems is described in WO 2010/097375 A3.
The distance between coils and the body of the patient is unavoidably greater for a remote body array than with local field coils. The strength of the signal to be measured is therefore weaker at the coil sites. The noise received from the body of the patient also decreases proportionally. The inherent noise of the high-frequency coils however remains the same and dominates the signal-to-noise ratio.
With a remote body array without further measures, the signal-to-noise ratio is much lower than with local field coils. As a result, either imaging quality decreases with an otherwise identical system or more time is required to obtain a full recording.
To improve the signal-to-noise ratio with remote body arrays, it is known to cool the high-frequency coils. The inherent noise of the high-frequency coils is proportional to their resistance and temperature. When the coils are cooled to cryogenic temperatures of less than 100 K, the noise is reduced by this alone. In the case of coils made of a normally conducting metal (e.g., copper), resistance also decreases with temperature, so a good signal-to-noise ratio may be achieved in a simple manner. In the case of superconducting high-frequency coils, which are cooled below the critical temperature TC, resistance and therefore inherent noise are negligibly small. The signal-to-noise ratio is thus only determined by the body of the patient and the environment of the coils.
A superconducting coil system for magnetic resonance tomography is known, for example, from U.S. Pat. No. 7,772,842 B2. High-temperature superconducting high-frequency coil arrays are used in U.S. Pat. No. 7,772,842 B2 to allow the imaging of individual body parts (e.g., chest, knee or hand). The cryogenic components are located in a vacuum chamber and are in contact with a chiller via heat-conducting apparatuses. Sapphire plates or heat pipes, for example, may be used for this purpose.
When used for remote body arrays, however, the large extension of the coil system poses problems for such cooling designs. For example, it is not always possible to provide reliable heat dissipation from the coils to the chiller using such heat-conduction apparatuses.